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Niels Bohr

Studies at Copenhagen

War and Manchester

Bohr left Copenhagen to seek J. J. Thomson at Cambridge.
While studying cathode rays, Thomson postulated the existence of
what he called "corpuscles," later to be renamed "electrons" by
H. A. Lorentz. This 1897 discovery ignited research worldwide,
including Bohr's own, and soon the traditional picture of the atom
as a solid ball was replaced by a picture that focused on the dynamic
between electrons and their positively charged counterparts, protons.
There was no doubt in Bohr's mind that his path lay with Thomson
at Cambridge, where he headed the illustrious Cavendish laboratory.

He hurried to Cambridge as soon as was feasible and arranged
a meeting. Thomson was cordial and showed some interest in the young
man's work. Language barriers made their communication difficult,
and Bohr tried to capture the revered man's attention by immediately
pointing to some errors he'd found in Thomson's work. While this
may or may not have changed Thomson's attitude, Bohr later recalled
and joked about his naiveté. Bohr presented Thomson with his dissertation,
and Thomson gave him some preliminary guidance on experiments to
be carried out. Not long after, Bohr saw that his work was not
showing much promise, and he found out that Thomson had still not
read his paper.

While working with Thompson, Bohr had the chance to hear Ernest
Rutherford speak. Rutherford had recently become famous with his
1911 discovery of the atom's nucleus. For years, Rutherford had
been shooting alpha particles at different targets, in order to
study atoms. Most of the alpha particles went through, but miraculously,
every once in a while a particle would be deflected back. This
discovery sent Rutherford into a long period of consideration, but
he emerged with revolutionary insight. He proposed that the atom
looked like a miniature solar system, with a massive center around
which electrons orbited. The majority of alpha particles passed
through the gold foil because of the vast space between the electrons
and the nucleus, but those that were deflected back must have struck
the small but mass-possessing nucleus.

Captivated by his brilliance and drawn to his personality,
Bohr soon decided to move to Manchester, and Rutherford accepted
him as a student. Although Rutherford was fundamentally an experimental
physicist and Bohr a theoretician, both developed great mutual
respect. When asked why he was able to make an exception for Bohr
from his general attitude toward theoreticians, Rutherford is said
to have responded, "Bohr's different. He's a football player!"
Rutherford's aversion to theorizing, however, may have held Bohr
back. Based on experimental evidence that arose in Rutherford's
lab, Bohr began toying with the idea of isotopes (an atom of an
element with the same atomic number but a differing atomic mass)
and radioactive displacement (the transformation of an element
to another element due to changes in atomic number). Rutherford
was not convinced and discouraged Bohr from advancing theories
without the appropriate evidence. Not long after, several scientists
independently began uncovering the evidence that would have proved
Bohr right, but Bohr never complained that he had not received
any credit, nor did he blame Rutherford for discouraging him.

Instead, he continued to push ahead with advances on Rutherford's
model of the atom. The fundamental difficulty he encountered was
that Rutherford's model proved unstable by classical standards.
According to Newtonian mechanics, the orbiting electron should
lose energy as it gave off radiation and eventually collapse into
the nucleus. Such a picture of course contradicted the stable physical
reality of the observable world. Earlier in the century, scientists
like Planck and Einstein had already begun to show the limitations
of classical physics in its picture of radiation and light. Through
extensive calculations they proved that thermal radiation and light
are not continuous. Instead, they are made up of individual packets
of energy, which they named "quanta." This radical picture of
matter unsettled the scientific community and was accepted only
gradually, but Bohr saw its relevance to the atomic world, which
seemed to require a new set of rules as well.

He became convinced that he could determine these rules
using the quantum of action (Planck's constant). He began toiling
over his calculations in the spring of 1912, working day and night
and determined to have a paper ready before his wedding date of
August 1. Drawing on the concept of quanta, Bohr attempted to show
that the atom could exist only in discrete states, each with its
own energy value. This theory later enabled him to account for
the series of lines in the spectrum of light emitted by the hydrogen
atom. In examining this hypothesis, Bohr not only advanced the
revolutionary quantum theory, but began to surmise answers to age-old
questions. Most notably, his theory had implications for the nature
of matter itself, and what gives the elements their distinctive
properties. This work was presented to the public in the form of
three articles, later referred to as the Trilogy, in the Philosophical
Magazine.

He left Manchester with these fresh insights, having spent
only four months there. He returned to Copenhagen for his wedding, and
after a honeymoon in Norway, England, and Scotland, he was prepared
to continue his work at the University of Copenhagen.